FIG. 1 illustrates a conventional heterojunction (HJ) photovoltaic (PV) diode 1 that includes a radiation absorbing mercury-cadmium-telluride (HgCdTe) n-type base layer 2. Not shown is an underlying electrically insulating and transparent substrate. A p-type HgCdTe region 3 forms a p-n junction 3a with the base layer 2. In that the photodiode 1 is typically one of a plurality of photodiodes provided as a two-dimensional array, the junction 3a is contained within an upstanding mesa structure that provides electrical isolation between adjacent junctions 3a. A passivation layer 4 is comprised of, by example, a layer of cadmium-telluride (CdTe) and functions to reduce surface electrical noise states. Contact metalization 5 is electrically coupled to the p-type region 3 and provides an interface pad for coupling to an indium bump 6. The indium bump 6 is provided to couple the photodiode 1 to external circuitry, such as a readout amplifier (not shown). The resulting hybrid assembly is further packaged, as required, for an intended application.
In operation, infrared (IR) radiation is incident on a bottom surface, or backside, of the photodiode 1. Radiation is absorbed within the base layer 2 and generates photocarriers. When suitably biased, minority photocarriers are collected at the p-n junction 3a and the resulting current is detected.
A conventional fabrication process for manufacturing the photodiode 1 uses deep trench etching (3-5 micrometers) to delineate the detector photodiode mesas. In that this etch is isotropic, lateral etching occurs at approximately the same rate as vertical etching. As a result, the mesa structure is tapered, with the top surface area being smaller than the bottom surface area. However, all subsequent processing for forming the contact metal and the indium bump must be accomplished on the relatively small top surface of the mesa. One disadvantage of providing a small indium bump interconnect is that the reliability of the subsequently formed hybrid assembly may be compromised.
At present, the minimum detector unit cell size is limited by the smallest indium bump size that can be placed on the top surface of the mesa. As can be appreciated, the fabrication of photodetectors having a small unit cell size is desirable in that denser arrays of photodetectors can be provided.
It is also known in the art to fabricate photodiodes by other than the mesa-etch technique described above.
For example, in U.S. Pat. No. 4,105,478 E. S. Johnson discloses the thermally driven diffusion of lithium into a p-type or an n-type HgCdTe body so as to form a p-n junction. However, the diffusion of dopants into a layer of semiconductor material forms a homojunction, which is more susceptible to leakage currents than is a heterojunction.
In U.S. Pat. No. 4,206,003 T. Koehler describes the implantation of an acceptor impurity, such as gold, phosphorus, antimony, or arsenic, into an n-type HgCdTe body to form a p-n junction. The implant is followed by a thermal anneal. In an article entitled "Development of HgCdTe LWIR Heterojunction Mosaics" (Proc. IRIS Detector, 1986, Vol. II, pages 251-260) C. C. Wang et al. describe a p-on-n LWIR detector that includes an undoped HgCdTe active layer having a p-type doped cap layer grown thereon. The structure is subsequently thermally annealed to convert the active layer to n-type, achieving a p-on-n structure. Instead of etching mesas, diodes are delineated by ion implantation to form lateral p-n junctions having short wavelength response. After implantation, passivation is applied.
However, in that both of these last-mentioned processes employ ion implantation, some damage to the semiconductor crystal structure occurs. This damage is a source of noise and leakage currents.
It is one object of this invention to provide a processing technique that enables a photodetector unit cell size to be made smaller than the unit cell size presently obtainable with the mesa-etch technique.
Another object of the invention is to provide a method for fabricating a planar photodetector that does not require either a thermal diffusion or an ion implantation of acceptor or donor species.
It is also an object of the invention to provide a processing technique that enables the size of the indium bump to be made significantly larger than the active area of an underlying photodetector, thereby increasing the contact area and the overall reliability of a hybrid assembly incorporating the photodetector.
A still further object of the invention is to provide an array of infrared radiation responsive heterojunction photodiodes having a substantially planar top surface to facilitate subsequent processing steps, such as the formation of indium bump interconnects.